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Commit06f5295

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Add single-item cache when looking at topmost XID of a subtrans XID
This change affects SubTransGetTopmostTransaction(), used to find thetopmost transaction ID of a given transaction ID. The cache is able tostore one value, so as we can save the backend from unnecessary lookupsat pg_subtrans/ on repetitive calls of this routine. There is a similarpractice in transam.c, for example.Author: Simon RiggsReviewed-by: Andrey Borodin, Julien RouhaudDiscussion:https://postgr.es/m/CANbhV-G8Co=yq4v4BkW7MJDqVt68K_8A48nAZ_+8UQS7LrwLEQ@mail.gmail.com
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‎src/backend/access/transam/subtrans.c

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@@ -54,6 +54,14 @@
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#defineTransactionIdToPage(xid) ((xid) / (TransactionId) SUBTRANS_XACTS_PER_PAGE)
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#defineTransactionIdToEntry(xid) ((xid) % (TransactionId) SUBTRANS_XACTS_PER_PAGE)
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/*
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* Single-item cache for results of SubTransGetTopmostTransaction(). It's
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* worth having such a cache because we frequently find ourselves repeatedly
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* checking the same XID, for example when scanning a table just after a
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* bulk insert, update, or delete.
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*/
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staticTransactionIdcachedFetchSubXid=InvalidTransactionId;
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staticTransactionIdcachedFetchTopmostXid=InvalidTransactionId;
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/*
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* Link to shared-memory data structures for SUBTRANS control
@@ -155,6 +163,13 @@ SubTransGetTopmostTransaction(TransactionId xid)
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/* Can't ask about stuff that might not be around anymore */
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Assert(TransactionIdFollowsOrEquals(xid,TransactionXmin));
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/*
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* Before going to the subtrans log, check our single item cache to see if
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* we know the result from a previous/recent request.
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*/
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if (TransactionIdEquals(xid,cachedFetchSubXid))
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returncachedFetchTopmostXid;
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while (TransactionIdIsValid(parentXid))
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{
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previousXid=parentXid;
@@ -174,6 +189,9 @@ SubTransGetTopmostTransaction(TransactionId xid)
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Assert(TransactionIdIsValid(previousXid));
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cachedFetchSubXid=xid;
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cachedFetchTopmostXid=previousXid;
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returnpreviousXid;
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}
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